Geoheritage

Amazing geology

Macquarie Island is a geological wonder. The main reasons for its listing are its unique geological values.

It is the only island in the world composed entirely of oceanic crust and rocks from the mantle - deep below the earth's surface.

Macquarie Island probably began as a spreading ridge under the sea with the formation of new oceanic crust somewhere between 11 and 30 million years ago. At some stage the spreading halted and the crust began to compress, squeezing rocks from deep within the mantle upwards like toothpaste from a tube. As the ridge grew it eventually became exposed above the ocean's surface about 600,000 years ago.

For the first time, rocks normally only occuring deep within the earth's mantle have become exposed on the earth's surface.

Unlike other subantarctic islands which have been shaped by glaciers, once Macquarie Island emerged, it has mainly been carved by marine processes such as wave action.

The geodiversity of Macquarie Island provides the foundation for the landforms, soils, plants and animals occuring here. It is an island of unique natural diversity, a site of major geoconservation significance and one of the truly remarkable places on earth.

A Marine Ridge

Notice the slope of the mountain as it drops towards the sea? This slope continues underwater at the same rate. As a result, the sea floor is very deep just a few kilometres offshore. Macquarie Island is actually on the top of a marine mountain ridge.

Generally, landmasses (such as Australia) have a continental shelf which slopes gently offshore. This makes the sea floor close to shore quite shallow. However as Macquarie Island is on a mountain ridge, the seafloor drops away dramatically.

Macquarie Island provides evidence of the rock types found at great depths in the earth's crust but also for plate tectonics and continental drift, the geological processes which have dominated the earth's surface for many millions of years.

Rocks from 6 km below the surface

Macquarie Island is a geological wonder, a site of world significance. Rocks outcropping on the north of the island have been forced up from about 6 km below the ocean floor, a unique exposure of rocks from the earth's mantle technically known as ophiolites. No drill hole has penetrated these depths and so these exposures provide a rare opportunity to gain an understanding of geological processes from some of the deepest rocks in the earth.

The southern part of Macquarie Island is composed of rocks which are typical of those currently forming along mid-oceanic ridges that contribute directly to the process of continental drift. These ridges occur on the ocean bed, where lava erupts out of long fissures running for hundreds of thousands of kilometres across the major ocean floors. They are only visible above the sea floor at a few places such as Macquarie Island and Iceland.

Equally impressive is the island's ongoing rapid rise out of the sea making it one of the most active geological regions in Australia. This is obvious from landslide and earthquake activity on the island which has produced a landscape controlled by prominent faults, many of which are clearly evident as long ridges with very steep slopes on one side. The sea has also been responsible for shaping most of the island as it has risen above the waves, more recently cutting distinct shore platforms and sea stacks. These active processes, have continued since the Macquarie Ridge appeared above sea level just over half a million years ago.

Why is Macquarie different?

Macquarie has a number of special features.

Firstly there are no other subantarctic islands which have been squeezed upwards from the oceanic crust to form an island like this. Most subantarctic islands, including Heard Island, initially developed as underwater volcanoes. Some are now exposed above the sea as a result of the accumulation of layers of lava. Prince Edward Island, the McDonald Islands and Iles Crozet also formed from submarine volcanoes while Auckland and Campbell Islands have volcanic origins but they were previously attached to continents and the continental crust.

Secondly the rocks of Macquarie Island which are in their original (oceanic) geological setting, are far less deformed than similar rocks elsewhere. For example, the Troodus complex of Cyprus, the Semail complex of Oman and the Bay of Islands complex of Newfoundland, although geologically similar to Macquarie Island, are more deformed. These rocks have also been "welded" to continental type rocks, a process in which the rocks are squashed, stretched or both.

There is still considerable geological debate over whether the rocks in the complexes actually originated from the oceanic crust. Some geologists feel that they have formed in entirely different geological settings such as the volcanic islands around the Pacific rim where oceanic crust is being forced, or subducted, below the adjacent continent.

Macquarie Island is in a totally different geological setting to these islands but is considered to be very close to its original oceanic setting. It has not been forced up against the edge of a continent.

As the geological origin of the island differs from that of other subantarctic islands, so does its landscape and the processes shaping it. The major difference is that most of the Macquarie landscape has been formed by marine erosion processes such as wave erosion as it has risen above sea level. In contrast many of the volcanic islands tend to develop in major explosive events when growth above sea level is very rapid and the only erosive effects occur around the coastal perimeter. It has also been argued that unlike other subantarctic islands, whose landscapes have been moulded by ice Macquarie, has not been glaciated.

The Birth of an Island

Macquarie Island is totally oceanic in origin all rock units having formed on or beneath the ocean floor.

It is estimated that between 30 million and 11 million years ago Macquarie Island first evolved in the Southern Ocean as a spreading ridge, a small version of the mid oceanic ridges which run for thousands of kilometres along the bottom of the large ocean basins. This is where basalt magma from deep within the earth flows up to the ocean floor and out through long fissures or volcanic vents forming new oceanic crust.

The southern parts of Macquarie Island are dominated by basalt which cooled under 2.5 km of water producing spectacular features such as pillow lavas . These are shaped like pillows and have a diameter of about 1 m. They cool very rapidly on coming into contact with sea water, forming round to oval shaped pillows which implode, or explode inwards. Evidence for this include a glassy external margin on the pillows which indicate rapid cooling and very little time for any crystal development.

About 10 million years ago the spreading stopped. Instead of moving apart areas on either side started to squeeze together forcing the oceanic crust and parts of the upper mantle, upwards. So there was a major reversal in geological processes and Macquarie Island started its 2.5 km journey to emerge above the sea surface about 600 to 700 thousand years ago.

Macquarie -- A Young Island

From a geological perspective Macquarie Island is very young. The oldest rocks dated are about 11.5 million years old, although they started forming between 11 and 30 million years ago. Due to the way the island evolved it is unlikely that older rocks would have been forced away from the centres where the basalt was escaping and where Macquarie Island started being forced upwards.

To put the age in perspective similar age rocks cover a very small portion of Tasmania mostly in the north of the State. A large portion of western Tasmania is underlain by rocks a thousand million years old. The oldest date in Australia is from a zircon crystal, found at North Pole in Western Australia, which is over 4000 million years old. These are some of the most ancient rocks on earth. They are not very widespread having been eroded by the elements or destroyed by continents being squashed together. Moon rocks are also around 4000 million years old but it has retained many of its original features, including a few extra craters through time, which are well preserved because there is no weathering and erosion.

Active Processes

In the last 600 to 700 thousand years, Macquarie has emerged above sea level and recent estimates suggests that since about 6000 years ago it has been rising at an average rate of about 0.8 mm per year. Evidence that the island has risen above the sea surface is provided by numerous old beaches or areas eroded by waves at altitudes ranging from 6 m to 400 m. The other large influence on the shape of Macquarie Island has been the extensive faulting which also contributes to earthquake activity. Macquarie Island experiences an earthquake of 7.5 on the Richter scale once every decade. There are many large and active faults which have shaped the coast, created fault dammed lakes and controlled the location of major landforms. Earthquakes trigger major landslip events, the scars of which cover many slopes.

Glaciation was thought by early geologists, around Mawson's time, to be the dominant landforming processes. They thought that ice may even have drifted from an adjacent continent to the west and basically scoured the top off the island. Up until the mid 1980s there was still support for the idea that ice was the dominant landforming process. More recent evidence such as beach deposits from just above sea level to high on the plateau seem to have discounted wide spread glaciation although in some places it is tempting to see cirque like features (small basins eroded by ice) near the tops of some peaks.

The most active processes today are associated with water freezing and thawing, landslips and wind which blows the finer particles from higher, exposed locations. When water solidifies to form ice it expands. This acts at various levels, from slowly wedging boulders apart to frost heaving exposed soil. This loosens the soil surface and fine material is removed by strong winds and water.

Wind is also the major contributor to the vegetation stripes and terraces which form in very exposed locations. The wind prunes the plants and removes finer material from the bare areas or the fine material is moved downslope by the almost continues precipitation from rain and snow.

On the terraces the risers (sloping parts) are generally vegetated while the treads (flat parts) are gravelled. Some people have proposed that they form as a result of the base of the riser being eroded by frost heave, wind and water while vegetation grows over the lip of the tread and so the whole flight of terraces moves slowly upslope. A few of the terraces are unvegetated suggesting that the vegetation has either died or that terraces may originally form as a result of a different process. For example freezing and thawing in the loose soil/substrate can gradually move material downslope as a terrace like feature, a process known as solifluction.

One of the more intriguing aspects of Macquarie Island is the general lack of creeks and river. Some lakes lack inflowing or outflowing creeks. This appears to be due to the highly fractured nature of the bedrock or the way in which the bedrock dips or slants. This results in water percolating quickly through the rock, perhaps ultimately re-emerging on the shore platform which skirts parts of the island. These are exceedingly boggy.

The bogs are composed of peat which have been reported as being up to 6 m deep and are thought to have formed over the last 5000 to 7000 years. This would suggest a very fast rate of organic accumulation or slow rate of decomposition in comparison to peats in other parts of the world. Rates are never constant, but on average it appears as if the lowland bogs on Macquarie have developed at about 10 times the rate of many other peatland areas in the world.

Peats dominate even sloping areas, blanketing parts of the island in what are known as blanket bogs which are a direct response to the wet, humid environment with very low evaporation. In many locations no fine soil material exists because it has been washed or blown away, leaving extensive cobble and boulder-strewn areas, which could appear like glacial debris and may be part of the reason early geologists thought the whole island had been glaciated.